As is well known, the recent semiconductor integrated circuitry technology is in rapid progress toward higher integration. In accordance with this propensity, the exposure light source used in the lithography process for semiconductor device manufacture is progressively reduced in wavelength. Nowadays, the lithography using an ArF excimer laser (193 nm) almost becomes the mainstream. To achieve further integration in the future, it is considered promising for the lithography to make a transition to F2 excimer laser (157 nm) or extreme ultraviolet (EUV). Since the F2 excimer laser lithography is now found to leave a number of technical problems to be solved, the transition to EUV lithography is deemed likely.
The EUV lithography is expected to use soft x-ray, especially a wavelength near 13 nm as the light source. Since there are available no materials having a high transmittance at such wavelength, a catoptric system is employed in the EUV lithography. In the system, reflection is assigned to a reflective multilayer film of silicon, molybdenum or the like deposited on a substrate, and several tens of percents of incident EUV light is not reflected, but reaches the substrate where it is converted into heat. In the EUV lithography using a light source of an extremely short wavelength as compared with the conventional lithography, even slight thermal expansion due to the heat that has reached substrates and other members in the lithographic optical system can adversely affect the lithography accuracy. Therefore, members including reflecting mirrors, masks and stages must be formed of low expansion materials. Also in the EUV lithography using a light source of a short wavelength, even slight irregularities on the member surface can adversely affect the lithography accuracy. Therefore, the surface topography or contour needs a high accuracy.
One of well-known low expansion materials is titania-doped quartz glass. Quartz glass can be reduced in thermal expansion by adding a certain amount of titania. However, prior art titania-doped quartz glass contains regions which are heterogeneous in structure and composition. Structurally and compositionally heterogeneous regions of one type are striae. In the case of titania-doped quartz glass, it is believed that striae are caused by changes of dopant titania amount. If titania-doped quartz glass containing strong striae is machined and polished into any member for use in the EUV lithography, the resulting member develops irregularities on its surface. It has not become possible to use prior-art titania-doped quartz glass as EUV lithographic members which must have a surface topography of the high accuracy required.
One known means for overcoming irregularities caused by striae is by polishing an EUV lithographic member and then selectively grinding off raised portions on the member surface using ion beam or the like. This means considerably increases the manufacture cost of members.
WO 03/76352 discloses a method of avoiding striae involving powdering titania-doped quartz glass containing striae and re-solidifying by Verneuil's method. With this method, there still remains a problem that granular structure or the like tends to generate irregularities on the member surface after polishing.
JP-A 2004-131373 discloses the preparation of titania-doped quartz glass by the sol-gel method. In general, the sol-gel method has problems such as difficulty to produce large-size ingots and crack susceptibility.
WO 02/32622 discloses a method of fusing a thin plate of titania-doped quartz glass without surface-exposed striae to a member with surface-exposed striae for thereby avoiding irregularities due to striae on the member surface. When titania-doped quartz glass is prepared by the so-called direct or indirect method of subjecting a silicon source gas and a titanium source gas to hydrolysis by oxyhydrogen flame, striae are often generated at intervals of less than several hundreds of microns and the striae are curved rather than flat. It is thus difficult to obtain titania-doped quartz glass without surface-exposed striae, leaving a problem from the productivity aspect as well. Even if a thin plate of titania-doped quartz glass without surface-exposed striae is used, the fusion of the thin plate must be followed by polishing, with an increased possibility that striae be exposed on the polished surface.
JP-A 2004-315351 discloses that EUV lithographic members having a high surface accuracy are obtainable from titania-doped quartz glass having a TiO2 concentration variation (ΔTiO2) less than or equal to 0.06% by weight. However, controlling only ΔTiO2 does not always succeed in producing EUV lithographic members having a surface topography with the required high accuracy.